Question: On the first 3 tests of her math class, Ashley got an average score of 88. What does she need on the next test to have an overall average of 91?
Explanation: Let her score on the next test be $x$ The sum of all of her scores is then $3 \cdot 88 + x$ The same sum must also be equal to $4 \cdot 91$ Solve: $x = 4 \cdot 91 - 3 \cdot 88 = 100$.